There is a desperate need for novel treatment strategies for pancreatic ductal adenocarcinoma (PDAC). The long term goal of this proposal is to develop strategies for durable cancer remission by targeting critical mediators of PDAC tumorigenesis and intrinsic drug resistance. Our central hypothesis is that activation of the oncogenic STAT3 pathway represents (a) is involved in intrinsic drug resistance and (b) constitutes a potential therapeutic target for PDAC. The overall objective is to understand the mechanism of constitutive activation of STAT3 in PDAC and determine its downstream biologic effects and clinical relevance. Using gain- and loss-of-function approaches, we will delineate the molecular mechanisms by which STAT3 contributes to PDAC tumorigenesis by defining key target genes and establishing the functional interaction between STAT3, EGFR and Src kinase. We will establish the in vivo effects of STAT3 inhibition using an orthotopic mouse model of PDAC and define the mechanism of interaction of STAT3 signaling within the TME utilizing novel imaging modalities to assess tumor angiogenesis, blood flow, hypoxia and tumor drug delivery in vivo. This will be accomplished by the following specific aims: Aim 1: To investigate the functional role of constitutively activated STAT3 on the pathogenesis and intrinsic drug resistance of PDAC. Hypothesis: The STAT3 transcriptional network is critical in the pathogenesis and chemoresistance of PDAC. Aim 1a will elucidate the mechanism of constitutive activation of STAT3 signaling in PDAC. Aim 1b will determine the biologic effects of STAT3 inhibition and silencing on fundamental target genes. Aim 1c will determine if STAT3 inhibition and silencing overcomes drug resistance in PDAC. Aim 1d will determine the prognostic significance of activated STAT3 in PDAC patients treated with neoadjuvant gemcitabine and erlotinib in a national clinical trial. Aim 2: To determine the contributory roles of EGFR, Src and gemcitabine interaction with STAT3 in overcoming chemoresistance in PDAC. Hypothesis: Combined targeted therapy with STAT3 will overcome redundant signaling pathways and feedback loops associated with STAT3-mediated chemoresistance in PDAC. Aim 3: To determine the mechanism of interaction of STAT3 signaling on the tumor microenvironment in PDAC. The tumor microenvironment (TME) plays a critical role in PDAC. Hypothesis: STAT3 inhibition will promote vascular normalization and depletion of stroma, resulting in enhanced drug delivery to the tumor, leading to improved therapeutic response. Successful completion of these specific aims will yield significant new knowledge regarding the mechanism of STAT3 activation in PDAC and will further delineate an increased understanding of the integration, functional relationships and collective roles of EGFR, Src and STAT3 which will be needed to derive effective, multitargeted therapy for PDAC. The aims proposed have direct translational relevance as they address an important molecular mechanism of drug resistance in PDACs and offer insight into how targeting tumor cells with STAT3 inhibitors may modulate the TME to improve sensitivity to chemotherapy.